Proper operation of an internal combustion engine requires varying air-fuel ratios to suit different operating conditions. Air enters the engine through a throttle body assembly mounted on the engine intake manifold. The air is subsequently mixed with fuel for use by individual cylinders in the combustion process.
The amount of air supplied to the engine during non-idle operation is controlled by a throttle plate located within the main air horn of the throttle body assembly. The throttle plate is mechanically interlocked to the vehicle accelerator pedal and is controlled by the amount of force exerted by the operator against the accelerator pedal.
During idle operation, no force is exerted against the accelerator pedal by the operator and the throttle plate is maintained in its normal, fully closed (or slightly open) position, restricting nearly all airflow around the throttle plate and through the air horn. The fuel requirements of the engine are drastically reduced. The minimum intake airflow required to maintain a stable idle RPM makes exacting control by a large throttle plate in the air horn extremely difficult. Therefore, the throttle body assembly is provided with an alternative (throttle-bypass) airway, bypassing the throttle plate and much smaller in size than the air horn. The throttle-bypass airway is equipped with an Idle Air Control (IAC) valve which provides for greater control over the amount of air flowing into the engine.
The IAC valve is controlled by a computer program (IAC control algorithm) contained within the vehicle on-board Electronic Control Module (ECM). Actuation of the IAC valve by the IAC control algorithm causes a pintle, protruding into the throttle-bypass airway, to move either further into or out of the airway. The position of the pintle in the throttle-bypass airway determines the amount of air permitted to pass through the airway and is varied as required to maintain a stable idle RPM.
An idle RPM fault condition occurs if the actual operating idle RPM is significantly higher or lower than a calculated, desired (commanded) idle RPM which varies based upon such factors as engine coolant temperature, vehicle speed and oxygen content in exhaust gases.
Detecting the existence of an idle RPM fault is essential to serviceability of the engine and ensuring proper operation of emission-related systems, in particular, the IAC System (including the IAC control algorithm, IAC valve, throttle-bypass airway and throttle plate). Serviceability is substantially enhanced by eliminating or verifying the IAC System as the source of the idle RPM fault.